Cyanobacterial blooms are on the rise worldwide, including in economically important, low nutrient, clear-water (LNCW) lakes where they have not historically been reported. Because cyanobacteria can access pools of sediment phosphorus (P) that are generally not available to eukaryotic phytoplankton, we hypothesize that cyanobacterial blooms can serve as internal drivers that work in concert with external drivers such as land-use and climate change to promote a shift from the LNCW state to a eutrophic state. However, empirical estimates of cyanobacteria-mediated internal P loading to LNCW lakes are lacking, especially relative to other sources of P. Here, we make this comparison for oligotrophic Lake Sunapee, New Hampshire, during summers 2005-2017. We first estimated internal recycling of P by cyanobacteria from measured rates of recruitment of the large colonial cyanobacterium Gloeotrichia echinulata from its dormant benthic stages together with lake hypsometry, surveys of available Gloeotrichia habitat, and literature values for Gloeotrichia P content. We then compared this biological recycling of sediment P to (1) external P inputs derived from stream inflow, climate, and precipitation-runoff relationships and (2) redox-mediated internal P recycling due to hypolimnetic anoxia estimated with the General Lake Model-Aquatic EcoDynamics model, a hydrodynamic-water quality model calibrated to Lake Sunapee.
Results/Conclusions
Our estimates of P loading via Gloeotrichia recruitment ranged from <0.1 to ~3 kg P per summer. By contrast, estimated annual loading via streamflow was ~800-2100 kg P, with only a small fraction of that input occurring during late summer (~1-3 kg for the 5 smaller inflows and 6-25 kg for the larger ones). In most years, the hypolimnion remained oxic, and thus there was likely no redox-mediated internal P loading. However, in years with hypolimnetic anoxia, high-end estimates suggest that redox-mediated internal loading could contribute ~5-50 kg P during the late summer. Thus, although P loading via Gloeotrichia does not contribute substantially to the whole-lake P budget on an annual scale, cyanobacteria-derived P can be a significant source to Lake Sunapee during late summer and early fall, particularly in years with low baseflow and an oxic hypolimnion. This P could fuel late-season phytoplankton blooms, especially given the warm conditions increasingly experienced during late August through September. We conclude that biologically-mediated internal recycling of sediment P, especially by cyanobacteria, should be considered alongside external inputs and chemically-mediated internal loading when building P budgets and setting external loading targets to protect water quality in LNCW lakes.